This application is a U.S. national phase of International application no. PCT/SE98/01769 filed internationally Sep. 30, 1998.
The present invention relates to fluorescent tubes. The inner tube is the actual fluorescent tube and is encapsulated in an outer tube. A heat insulating air layer is obtained which surrounds the inner tube. In this way, an improved light yield is obtained from it in installations in cold environment.
Fluorescent tubes of the above-mentioned type are often used for illuminated signs, especially double-sided such. These have two display sides which are arranged parallel with respect to each other with a relatively small mutual distance between them. Fluorescent tubes are arranged between the display sides, usually in the form of a group of straight, mutually parallel fluorescent tubes. In this connection it is desirable that the display sides have an as even illumination as possible in order to avoid disturbing bands of light in the appearance of the sign. In order to achieve this, the fluorescent tubes can be arranged close to each other which, however, entails high costs. A normal compromise is there-fore to arrange the fluorescent tubes with such a large mutual distance that the illumination intensity of the darkest parts of the sign sides is approx. 50% of the illumination intensity of the lightest parts.
The object of the present invention is to improve fluorescent tubes of the type mentioned in the introduction in order to be able to give the sides of the sign a more even illumination.
According to a preferred embodiment the transmission characteristics of the diffuser layer in the circumferential direction of the tube varies from a high value at the longitudinal edge parts of the diffuser layer to a low value at the centre part situated in the middle between the edge parts of the layer. Variations in the light intensity in different directions from the fluorescent tube then become less. Further improvement can be obtained if the reflective ability of the diffuser layer varies in the reverse manner to the transmission ability.
The two diametrically situated diffuser layers spread the light transmitted therethrough but also prevent the transmission of a part of the thereon incident light flux from the inner tube. At the same time each diffuser layer reflects a part of the incident light flux towards the diametrically opposite half of the outer tube. The inner tube prevents a part of this reflected light flux from reaching the other half of the tube. This screening effect is greatest at the centre part of the diffuser layer, which is favorable because it is desirable to attenuate the light intensity in this direction. In this way the light intensity will be more even in different directions from the fluorescent tube and thereby the illumination will be more even on planar surfaces, e.g. the sides of signs, arranged parallel with a central plane between the diffuser layers. For example, it has been noted that when fluorescent tubes according to the invention were mounted in a sign which with prior art fluorescent tubes gave an illumination intensity of 50% at the darkest parts compared with the lightest, at the darkest parts an illumination intensity of 80% to 90% of that of the lightest parts was obtained.